Add new cfg module
    
    As discussed on the mailing list and described here:
    
      http://wiki.openstack.org/CommonConfigModule
    
    The module implements an API for defining configuration options and
    reading values for those options that a user may have set in a config
    file or on the command line.
    
    The module will be part of openstack-common and glance will consume
    it from there once openstack-common makes a release with an API
    compatibility commitment.
    
    Change-Id: Ideedf77e5e16fa4e6ca742cfb49f5539c5a4a746